Aircrafts have power requirements which are not related to propulsion. These can include ground services, such as providing air to the cabin, providing electrical power for the aircraft's system, and assisting with main engine start. These ground services are generally provided by one or more of the main propulsion gas turbine engines of the aircraft, or an auxiliary power unit (APU) on larger aircraft.
The main propulsion gas turbine engines operate most efficiently while providing propulsion during flight. It is therefore not very efficient to use these gas turbine engines as a source of motive or electrical power for aircraft ground services. For their part, APUs are rarely used during flight and typically serve only to provide ground services. APUs therefore must have systems which duplicate those already present in the main propulsion gas turbine engines. This redundancy increases complexity and costs. Furthermore, APUs are gas turbine engines themselves, and thus produce appreciable noise and combustion emissions when in operation.